Mark R Chapman

Changes in sea surface hydrology associated with Heinrich event 4 in the North Atlantic Ocean between 40° and 60°N

Abstract The changes in distribution of sea surface temperature and salinity in the North Atlantic between 40 and 60°N were reconstructed for the time interval between 40 and 30 kyr BP, which includes the large iceberg discharge event associated with the deposition of Heinrich layer 4. We found that the meltwater input during deposition of Heinrich layer 4 resulted in a 1–2 kyr temperature decrease of about 2°C and a salinity decrease in the range of 1.5‰–3.5‰ between 40 and 50°N. Sites above 50°N did not experience significant salinity variations. A much larger area was affected by the reduction in sea surface temperature. The amplitude of the sea surface temperature shift was, however, much smaller than the atmospheric temperature changes over Greenland at GISP and GRIP sites.

Global ice-volume fluctuations, North Atlantic ice-rafting events, and deep-ocean circulation changes between 130 and 70 ka

Multiproxy data from North Atlantic deep-sea sediment core NEAP18K provide a detailed record of climate through oxygen isotope stage (OIS) 5. Seven distinct, large-scale episodes of ice rafting (C25–C19) were identified between 126 and 70 ka. Global ice-volume reconstructions, based on high-resolution benthic δ 18 O records, indicate that major ice-rafting events were not confined to ice-volume maxima at OIS 5d and 5b, but also occurred during periods of ice-sheet growth and disintegration. However, iceberg discharges were restricted to times when sea levels were 40–65 m below present values. Ice-rafting episode C25, the first large-scale cooling of mid-Atlantic surface waters after the last interglacial, occurred during the gradual buildup of continental ice sheets at the OIS 5e-5d transition. Major ice-sheet collapses allied to ice-rafting events C24 and C21 were associated with rapid sea-level increases of 20 and 40 m, respectively. Suborbital climatic fluctuations in the NEAP18K sedimentary record, denoted by prominent 7.5, 4.5, and 3 k.y. cyclicities, appear to correlate with both Greenland atmospheric temperatures and changes in thermohaline circulation patterns, inferred from benthic δ 13 C values, and hence provide clear evidence of a highly interconnected North Atlantic climatic regime during OIS 5.

Millennial-scale fluctuations in North Atlantic heat flux during the last 150,000 years

Abstract Surface water and deep ocean palaeoclimate records obtained from North Atlantic core SU90-03 (40°N, 32°W) exhibit pronounced fluctuations at sub-orbital time scales. Surface temperature variability in the mid-latitude Atlantic over the last 150 kyr is dominated by forcing at precession (21 kyr) and semi-precession harmonics (11 kyr). Abrupt changes in sea surface temperatures show that the northward transport of heat by the North Atlantic Current was curtailed during ice rafting events. Sea surface temperatures were about 12°C colder than modern values during the most intense cooling episodes associated with major ice rafting events (Heinrich events 1, 4, 6 and 11) and 10°C below modern values during the later part of isotope stage 3 (40–30 ka). During these cold intervals, the North Atlantic Current was displaced to south of 40°N as the cold meltwaters penetrated southwards into what is now the subtropical ocean. These changes in surface ocean heat exchange were most probably allied to a latitudinal migration and increase in zonality of atmospheric circulation patterns. Concurrent shifts in IRD concentration, sea surface temperatures and benthic δ 13 C values provide evidence of coupling between sea surface processes and the deep-water circulation, and suggest that decreases in surface temperature and salinity during ice rafting events culminated in a significant reorganisation of North Atlantic Deep Water production.

Faunal and alkenone reconstructions of subtropical North Atlantic surface hydrography and paleotemperature over the last 28 kyr

Two techniques for estimating past variations in sea surface temperature (SST) have been used to investigate climatic change in Biogeochemical Oceanic Flux Study (BOFS) core 31K (19°N, 20°W) from the eastern subtropical Atlantic. High-resolution SST records for the last 28 kyr have been produced using planktonic foraminiferal assemblages, based on the Imbrie-Kipp transfer function technique, and the UK′37 index derived from abundances of C37 alkenones biosynthesized by prymnesiophyte algae. Modern observations suggest that these indices reflect particular hydrographic conditions in the upper ocean: the UK′37 index corresponds to the temperature at the time of maximum coccolith productivity, typically late spring-early summer in the study area today, whereas the faunal transfer function is calibrated for seasonal maximum and minimum temperatures. In general, the faunal and biomarker paleotemperature records display comparable SST variations during the last glacial and deglacial, but although the overall trends are similar, differences exist in the magnitude and timing of these temperature changes. Most notably, the faunal Twarm and UK′37 SST estimates diverge by 3°C between 8 ka and 6 ka, and this offset persists through the late Holocene. This difference cannot be adequately explained by uncertainties associated with either the calibration data sets or fluctuating preservation levels. We therefore propose that the deviation in SST estimates is linked to a switch in the seasonal timing of maximum coccolith production from the summer in the glacial ocean to the late spring-early summer in the modern ocean. Our results suggest that a dual approach to SST estimation based on faunal and biomarker proxies can provide a valuable means of evaluating mixed layer and productivity changes associated with the movement of oceanographic frontal zones during the late Quaternary.

Deglacial surface circulation changes in the northeastern Atlantic: Temperature and salinity records off NW Scotland on a century scale

Sea surface temperature and salinity estimates reconstructed from a core collected on the Barra Fan, northwest Scotland (56°43′N, 09°19′W; water depth 1320 m), show a series of rapid oscillations during the last deglacial period that are very similar to those observed in the δ18O records from Greenland ice cores. These records indicate that the transport of heat and salt toward the Nordic Seas was highest during the Bolling period. This “superconveyor” weakened after the Bolling, probably as a consequence of increased meltwater flux reducing the oceanic salt content, as suggested by the Barbados sea-level record. Evidence for a phase of ice rafting during the Allerod is presented for the first time from this latitude in the northeast Atlantic. The Younger Dryas stadial, resolved here at a century/decadal scale, is characterized by very rapid oscillations in temperature and salinity, indicating that warm, relatively saline waters repeatedly displaced cool polar waters at this latitude. These observations attest to the inherent instability of the deglacial climate system.

Low-latitude forcing of meridional temperature and salinity gradients in the subpolar North Atlantic and the growth of glacial ice sheets

Sea-surface temperature (SST) and salinity (SSS) records obtained from two deep-sea cores have been used to reconstruct meridional gradients in the mid-latitude North Atlantic over the past 40 k.y. During the buildup of ice sheets toward the last glacial maximum, SST and SSS gradients between 40° and 50°N increased by 6–8 °C and ∼2‰, respectively. This trend represents a two-fold increase in surface-water gradients between 40 and 20 ka and strongly overprints the signal of high-frequency climatic oscillations. We attribute the gradual intensification of meridional SST and SSS gradients to a precession-driven increase in advection within the glacial North Atlantic Current resulting from changes in low-latitude insolation and seasonality. A strongly zonal ocean circulation pattern, maintained by greater advection and enhanced heat storage within the subtropical ocean at the last glacial maximum, persisted until ca. 20 ka, when a decrease in glacial SST gradients, coincident with a reduction in low-latitude climate forcing, marked the initiation of the last deglaciation.

What level of resolution is attainable in a deep-sea core? Results of a spectrophotometer study

A Minolta CM-2022 spectrophotometer has been used to characterize downcore fluctuations in sediment lightness and color in core NEAP15K, a 7-m core collected from the northeast Atlantic Ocean. High-resolution data series, measured using a 4-mm-diameter measurement spot and a 1-cm sampling interval, were generated along two independent tracks down the core to investigate the statistical significance of fluctuations across the 2–20-cm-depth range. This small-scale variability is characterized by abrupt changes in the lightness and color of the sediment that are several orders of magnitude greater than the instrumental precision. Our results establish that significant information is preserved at the 1-cm scale in spite of bioturbation effects. These findings demonstrate that high-resolution studies using conventional paleoclimatic proxies have the potential to recover meaningful century-scale climate records in regions of the ocean where sedimentation rates exceed 10 cm per thousand years. The coherency of these downcore records also implies that the spectrophotometer is a powerful instrument for establishing precise centimeter-scale stratigraphic correlations between cores.

Coherent deep flow variation in the Iceland and American basins during the last interglacial

The first direct evidence that the deepflow speed of North Atlantic Deep Water (NADW) in the South Iceland and North AmericanBasins declined dramatically over a period of a few hundred years at about 120,000 years ago is presented. The flow rate inferred from silt grain-size then rose into the subsequent cool period (stage 5d). These changes are clear and synchronous in records from both locations. These circulation events are recorded less markedly in chemical and isotopic proxies for the nutrient status of water masses. The congruence of flowvariation and hydrography at the two separated sites suggests they are primarily controlled by variation in the deep thermohaline circulation.

Late Pliocene climatic change and the global extinction of the discoasters: an independent assessment using oxygen isotope records

Abstract High-resolution records (2–7 kyr) of Upper Pliocene Discoaster abundances obtained from six ODP/DSDP sites are assessed independently using oxygen isotope stratigraphy. Four Atlantic Ocean sites (DSDP Sites 552 and 607, and ODP Sites 659 and 662) comprise a transect from 56°N to 1°S and provide a record of latitudinal variations in Discoaster biogeography. Low-latitude sites in the Atlantic (ODP Site 662), Pacific (ODP Site 677), and Indian (ODP Site 709) oceans provide additional information about variability in Discoaster abundance patterns within the equatorial region. A common chronology, based on the astronomical time scale developed for ODP Site 677, has been established for all the sites. By integrating oxygen isotope data and Discoaster abundance records at each site we are able to independently evaluate the temporal and spatial distribution of D. brouweri and D. triradiatus in the 500 kyr prior to the extinction of the discoasters near the base of the Olduvai subchron. Major decreases in abundance are evident during some of the more intense late Pliocene glacial events. In particular, glacial isotope stages 82, 96, 98 and 100 are associated with distinct abundance minima. At these times, large-scale changes in surface hydrographic conditions appear to have suppressed Discoaster numbers on a global scale. The increase in abundance of D. triradiatus, which precedes the extinction of the discoasters by around 200 kyr, may also be related to the intensification of environmental pressures that accompanied the build-up of Northern Hemisphere ice sheets during the late Pliocene. In spite of contrasting geographic and oceanographic settings, the various D. brouweri and D. triradiatus records are remarkably similar. This demonstrates that the acme and extinction events are excellent biostratigraphic datums. The simultaneous extinction of D. brouweri and D. triradiatus at 1.95 Ma were synchronous events at both a regional scale within the Atlantic, and on a global scale between the three major oceans. However, the start of the D. triradiatus acme appears to have been diachronous, occurring some 40 kyr earlier in the Atlantic than in the Indo-Pacific, and hence the stratigraphic usefulness of this datum is regional rather than global.